332 ENVIRONMENT AMONG ANIMALS AND PLANTS 



is every reason to believe that it is not the fact that horse-ancestry 

 is in one case traced through the father and in the other case 

 through the mother that gives rise to the difference ; the difference 

 can only be attributed to the fact that in one case the period 

 before birth is passed within a mother of one species and in the 

 other case is passed within a mother of another species. 



4. We are now in a position to discuss what is meant by the 

 term * normal environment '. There is a more or less definite 

 range of variations of the environmental stimuli to which species 

 in a state of nature are subject. The range may differ greatly 

 from species to species, but remains more or less constant for any 

 species. So long as the variations fall within this range, the 

 environment may be described as normal. Such variations will 

 be followed by different responses on the part of similar germinal 

 constitutions, and the variations among the members of any 

 species under a normal environment are due to the combined 

 influence of differences in the environment and of differences in 

 the germinal constitutions. From time to time in a state of 

 nature organisms are subject to variations of the environmental 

 stimuli which fall outside the normal range, and there are thus 

 produced extreme modifications similar to those which can be 

 experimentally induced. Thus Gemmill on examining a large 

 number of fish embryos found monsters with a single eye very 

 similar to those experimentally produced by Stockard.^ 



It may next be observed that there is a marked difference in 

 the degree to which sessile organisms on the one hand and free- 

 living organisms on the other hand respond to differences in the 

 normal environment. The former are much more susceptible 

 to differences — or at least to certain differences. Sessile organisms, 

 for instance, differ from one another very greatly in form and 

 such variations are known to be chiefly due to differences in the 

 environmental stimuli. . Free-living organisms do not vary in 

 this manner. The reason is fairly clear. All species are adapted 

 to a particular niche in nature, and among the former the mode 

 of adaptation of necessity takes the form of susceptibihty to 

 surrounding conditions ; a tree or a sponge must be able to adapt 

 itself to its actual surroundings. A free-living organism is, on 

 the other hand, adapted by its specific form to its niche in nature 



1 Gemmill, Teratology oj Fishes, p. 44. The cyclopia found was attributed by 

 Gemmill only partly to environmental causes. 



